Only send the FrameSinkId to client when it is necessary

ui/aura/mus/mus_context_factory.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_AURA_MUS_MUS_CONTEXT_FACTORY_H_
 #define UI_AURA_MUS_MUS_CONTEXT_FACTORY_H_
 
 #include <stdint.h>
 
 #include "base/macros.h"
@@ skipping 19 matching lines @@
 class AURA_EXPORT MusContextFactory : public ui::ContextFactory {
  public:
   explicit MusContextFactory(ui::Gpu* gpu);
   ~MusContextFactory() override;
 
  private:
   // Callback function for Gpu::EstablishGpuChannel().
   void OnEstablishedGpuChannel(base::WeakPtr<ui::Compositor> compositor,
                                scoped_refptr<gpu::GpuChannelHost> gpu_channel);
 
-  void OnCompositorFrameSinkAvailable(
-      base::WeakPtr<ui::Compositor> compositor,
-      std::unique_ptr<cc::CompositorFrameSink> compositor_frame_sink);
-
   // ContextFactory:
   void CreateCompositorFrameSink(
       base::WeakPtr<ui::Compositor> compositor) override;
   scoped_refptr<cc::ContextProvider> SharedMainThreadContextProvider() override;
   void RemoveCompositor(ui::Compositor* compositor) override;
   double GetRefreshRate() const override;
   uint32_t GetImageTextureTarget(gfx::BufferFormat format,
                                  gfx::BufferUsage usage) override;
   gpu::GpuMemoryBufferManager* GetGpuMemoryBufferManager() override;
   cc::TaskGraphRunner* GetTaskGraphRunner() override;
   void AddObserver(ui::ContextFactoryObserver* observer) override {}
   void RemoveObserver(ui::ContextFactoryObserver* observer) override {}
 
   ui::RasterThreadHelper raster_thread_helper_;
   ui::Gpu* gpu_;
   base::WeakPtrFactory<MusContextFactory> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MusContextFactory);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_MUS_MUS_CONTEXT_FACTORY_H_